Field of the Invention
The invention relates to a turbine wheel in a turbine engine, and also to a turbine engine including a turbine fitted with such a wheel.
Description of the Related Art
In the prior art, a turbine wheel comprises a disk including in its outer periphery a plurality of teeth that are regularly distributed around the axis of the wheel and that between them define slots for axially mounting and radially retaining blade roots.
Each blade root has a circumferentially-extending platform with an airfoil extending radially outwards from the platform. The blade platforms are arranged circumferentially end to end radially facing the teeth of the disk, and they form an inner wall defining the inner surface over which there flows a stream of hot gas leaving a combustion chamber arranged upstream from the turbine.
On either side of the blade roots there are formed circumferentially-projecting portions that are arranged immediately over the teeth of the disk. These projecting portions, also referred to as rims, serve in particular to limit the extent to which hot gas can be reinserted between the platforms and the teeth of the disk. These rims also serve to limit tilting movements of the blades in a circumferential direction by coming to bear against the teeth of the disk.
At their free radial ends, the blades present respective transverse elements or “outer platforms” that serve in particular to define the outside of the flow passage for the gas stream passing through the turbine.
The outer platform of a blade has an upstream edge and a downstream edge extending perpendicularly to the flow direction of the gas stream. These upstream and downstream edges are connected together via two side edges via which the outer platform of the blade makes contact with the outer platforms of two adjacent blades of the rotor wheel.
In general, with metal blades, these side edges have a so-called Z profile, i.e. each of them has two axial portions connected together by a portion that is substantially transverse. In order to damp the vibration to which they are subjected while the turbine is in operation, it is known to mount the blades on the disk so that they are subject to prestress in twisting about their main axes. At the outer platform of any given blade, this twist prestress gives rise to the transverse portions of the outer platform of the blade being put into contact with the transverse portions of the outer platforms of the adjacent blades. The contact and friction forces as generated in this way via the outer platforms of the blades serve to dissipate the vibratory energy that results from the turbine operating.
Such vibration damping is nevertheless not applicable to rotor wheels having blades that are made of composite material. In particular, for a blade made of ceramic matrix composite (CMC) material, the stresses generated by twisting the blade are too great compared with the capacities of the composite material.
In order to solve those problems, the Applicant has proposed in its application WO 2011/104457 to mount vibration dampers between the outer platforms of adjacent blades made of composite material in a rotor wheel.
That solution is particularly advantageous for blades made of composite material. Nevertheless, it is difficult if not impossible to make anti-tilting rims for such blades. Composite material blades are made by superposing a plurality of layers of material, and adding such rims would require adding additional layers of material that would then need to be machined, which is lengthy, expensive, and difficult to do with composite materials.
Thus, in the absence of rims, the blades are free to tilt about respective axes passing through their blade roots, and that can lead to separation of dampers mounted between the outer platforms of adjacent blades, so that the blades no longer present good mechanical behavior when faced with vibration. Furthermore, the absence of rims leads to a longitudinal cavity between the teeth and the radially facing platforms, thus allowing hot gas to recirculate over the teeth of the disk and greatly shortening the lifetime of the disk and of the entire rotor of which it forms a part.
In order to limit circumferential tilting of the blades, it is known to mount spacers between the blade roots and the bottoms of the slots, such spacers exerting an outwardly-directed radial force in order to press the blade roots against the flanks of the slots.
Nevertheless, such spacers are difficult to make because of the small gap that exists between the root of each blade and the bottom of the corresponding slot, which means that each spacer must be machined individually. Furthermore, such spacers can become deformed in operation, which means that they no longer guarantee permanent radial pressure between the blade roots and the flanks of the slots. Finally, adding an additional part complicates the operations of assembling the rotor wheel and does not provide any solution to the problem of hot air recirculating between the platforms and the teeth of the disk.
In order to limit fretting phenomena between the blade roots and the disk, it is known to mount respective protection members on the blade roots. Interposing such a protection member that presents lower resistance to wear than do the blade root and the disk, makes it possible to preserve the blade root.